1. Field of the Invention
This invention relates to a video signal reproducing apparatus and more particularly to a video signal reproducing apparatus arranged to obtain a video signal by performing a continuous reproducing operation on one and the same recording track containing one field portion of a video signal which is arranged to have one frame thereof consist of two fields.
2. Description of the Prior Art
This specification deals by way of example with magnetic sheet reproducing apparatuses of the kind arranged to obtain a still picture by continuously reproducing a video signal which corresponds to one field portion of a television signal and is recorded in one of the circular recording tracks formed on a magnetic recording sheet.
In apparatus of this kind, a television signal duly consisting of first and second field portions (hereinafter called a full-field signal) is arranged to be obtained from one and the same field portion of the video signal. Compared with an ordinary full-field television signal, resolution in the vertical direction of a signal reproduced by this arrangement becomes 1/2 of the ordinary, full-field television signal. A conventional practice in solving this problem has involved obtaining a video signal (hereinafter called an averaging video signal) by taking an average level of a video signal obtained during two adjoining horizontal scanning periods within one reproduced field portion of the video signal. Then, a signal processing operation is performed to improve the resolution in the vertical direction in such a manner that the scanning lines on a picture resulting from the average video signal are arranged to be between scanning lines on a picture resulting from the video signal of the two adjacent horizontal synchronizing periods.
FIG. 1 of the accompanying drawings shows by way of example a video signal reproducing apparatus using the conventional method. The video signal reproducing apparatus of this example is arranged to reproduce one field portion of a color picture signal recorded in one circular recording track on a magnetic sheet 1. In the recording method of the apparatus, a brightness signal (hereinafter called a signal Y) is FM modulated in such a way as to be located within a relatively high frequency zone. As for color signals, two color difference signals B-Y and R-Y are arranged to be line sequential. The line sequential color difference signals are FM modulated to have them within a relatively low frequency zone. A color video signal obtained by mixing these signals is recorded. The symbols B and R shown above respectively represent a blue signal B and a red signal R. Again referring to FIG. 1, the illustration includes a magnetic head 2 which is arranged to reproduce a video signal by repeatedly tracing, the above-stated circular recording track accordingly as the magnetic recording sheet 1 rotates. The video signal obtained from the head 2 is supplied via a pre-amplifier 3 to a high-pass filter 4 (hereinafter called HPF for short), a low-pass filter 5 (hereinafter called LPF) and a drop-out detection circuit 6 respectively.
At the HPF 4, an FM modulated brightness signal is separated and is demodulated at an FM demodulator 7. The demodulated brightness signal is supplied to a drop-out compensation circuit 8. A delay line 9 (hereinafter called a 1 HDL) which is arranged to effect a delay by one horizontal scanning period. A signal which passes through this 1 HDL 9 and a signal which does not pass through the 1 HDL 9 are supplied to a switch 10. The switch 10 is connected to one side A thereof when a drop-out is detected and to another side B with no drop-out detected. When a drop-out exists in the demodulated brightness signal, the signal is thus replaced with a signal preceding it by one horizontal scanning period (hereinafter called 1 H), so that the drop-out can be compensated for.
The output of the switch 10 is thus supplied in the above-stated manner to a circuit 11 which is arranged to increase resolution in the vertical direction. FIG. 2 is - a timing chart which schematically shows signals (a)-(d) obtained at different elements disposed within the resolution increasing circuit 11. A reference numeral 12 included in the circuit 12 denotes a 1 HDL. The output of the 1 HDL 12 (see (b) in FIG. 2) and a signal which does not come through the 1 HDL 12 are added by an adder 13. Incidentally, in cases where a television signal is to be obtained by continuously reproducing one field portion of a video signal, if, for example, the one field portion of the video signal is composed of an odd multiple of 1/2 H, the period of the horizontal synchronizing signal comes out of order at a joint between one field portion of the video signal and another. For example, in the case of a video signal having one field portion thereof consisting of 265.5 H pursuant to the NTSC system, the horizontal synchronizing signal deviates by 1/2 H. In view of this, the circuit 11 includes a 1/2 HDL 14 which is arranged to compensate for such a 1/2 H deviation that takes place when one field portion of a video signal is continuously reproduced. The circuit 11 further includes a switch 15 which is arranged to alternately produce the output signal of the adder 13 (see (d) in FIG. 2) and the output signal of the 1/2 HDL 14 (see (c) in FIG. 2) by effecting a switch-over between them for every field period. As a result, the video signal will include the signal shown at (d) in FIG. 2 for a first field and the signal shown at (c) in FIG. 2 for a second field. A picture thus obtained will have a scanning line of the first field which is obtained by averaging them interposed between two adjacent scanning lines of the second field. The output terminal of the adder 13 is connected to the switch 15 via a 1/2 voltage divider 13a provided for level adjustment. The brightness signal produced from the switch 15 is supplied to a color encoder circuit 16 for making it into a television signal of the NTSC system or the like. The television signal thus obtained is arranged to be produced from an output terminal 17.
Meanwhile, the FM modulated line sequential color difference signals separated at the LPF 5 are demodulated at the FM demodulator 17. Then, the demodulated signals are corrected for the above-stated 1/2 H deviation by a 1/2 HDL 18 and a switch 19. The signal then passes through a 1 HDL 20 and both the signals which do and do not pass therethrough are respectively supplied to switches 21 and 22. These switches 21 and 22 are arranged to shift their positions at every H. When the switch 21 produces the signal which has passed through the 1 HDL 20, the switch 22 produces the signal which has not passed through the 1 HDL 20. Conversely, when the switch 21 produces the signal not having passed through the 1 HDL 20, the other switch 22 produces the signal which is produced from the 1 HDL 20. Each of the switches 21 and 22 thus produces only one color difference signal. The color difference signal obtained via each of the switches 21 and 22 is supplied to the color encorder circuit 16. The illustration of FIG. 1 further includes a detector 30 which detects the circuit phase of the sheet 1 and a circuit 31 which is arranged to produce a switching control signal for controlling the switches 15, 19, 21 and 22.
The apparatus which is arranged as described in the foregoing is capable of enhancing the resolution in the vertical direction. However, the arrangement of the apparatus necessitates provision of many delay means such as the 1 HDL's and the 1/2 HDL's as shown in FIG. 1. The prior art arrangement thus has presented a problem in respect to practicability. Further, it has been another problem with such arrangement that the apparatus inevitably becomes cumbersome.